In lifts it is usual to provide safety functions in order to monitor the travel behaviour of the lift car (or of the lift cage) in order that accidents can be avoided.
For this purpose, for example, a device for monitoring retardation is provided. In the lift shaft there are mechanical buffers on the pit and top side, which are used to brake the lift car in the event of an emergency. However, one precondition for this is that the speed of the lift car does not exceed a predetermined limiting value. In order to achieve this, if the limiting value is exceeded, the braking device for the lift car must be set operating early enough in order that sufficient braking, which takes place in accordance with a braking ramp, can be carried out in such a way that the buffers can perform the residual braking. For this purpose, it is known for the device for monitoring retardation to comprise electromechanical switches arranged in the shaft at an appropriate point and coupled to a safety relay module and, when they are reached during a lift travel in the direction of the buffers and if the limiting value for the speed is exceeded, for the cabin braking to be triggered and, at the same time, the drive to be switched off.
Furthermore, it is known to provide a mechanical device for limiting the speed of the lift car, which is used to trigger a braking system fixed to the lift car and under the control of centrifugal force when a predetermined desired value of the speed of the lift car is exceeded.
In addition, it is known to provide a device for approaching and, if necessary, readjustment with open doors, a bridging device being provided for electromechanical door contacts which monitor whether the shaft door is closed. In the case of high buildings with very many storeys, in order to save travelling time, provision is made to begin to open the doors at a specific distance before the selected storey position is reached, that is to say as the latter is approached, so that they are open when this position is reached. If a relatively large load is introduced into a lift car or is removed from the latter, the floor of the lift car moves in relation to the floor level of the corresponding storey at which the lift car is standing. In order that readjustment can be carried out to align the floor level of lift car and storey with the doors open, it is likewise necessary to bridge the door contacts in an appropriate manner. The same is correspondingly true of the case in which adaptation of the floor height of the lift car to a loading ramp height, for example of an HGV, is to be carried out, what is known as ramp travel control.
Mechanically based safety devices of this type do not permit continuous monitoring, only that at a point.
For example, EP 0 694 792 B1 and EP 1 030 190 B1 disclose a device for registering the position of a lift car, there being in the lift shaft an acoustic signal conductor having a predetermined, uniform velocity of propagation of sound, while the lift car has a signal injector for injecting a clocked acoustic signal into the acoustic signal conductor. Arranged at both ends of the acoustic signal conductor are signal extractors, which are connected to an evaluation unit for determining the propagation time difference of the injected acoustic signal from the injection point to the signal extractors and for generating a signal representative of the instantaneous position of the lift car in the lift shaft.